Silicon carbide (SiC) is excellent in heat resistance and mechanical strength and is physically and chemically stable. Therefore, silicon carbide is attracting attention as an environment-resistant semiconductor material. In addition, in recent years, there have been increased demands for an SiC single crystal substrate, as a substrate for a high-frequency high-voltage resistant electronic device, etc.
In a case where electric power devices, high-frequency devices, etc., are to be manufactured by using an SiC single crystal substrate, it may be usually performed in general to epitaxially grow an SiC thin film on a substrate, by using a process called a chemical vapor deposition process (CVD process), or to directly implant thereinto a dopant by using an ion implantation process. In the latter case of the ion implantation process, annealing at a high temperature may be required after the implantation, and for this reason, the formation of a thin film using an epitaxial growth may be employed frequently.
In the case of forming a device on an epitaxial film, in order to stably produce a device as designed, the film thickness and doping density of the epitaxial film, particularly the wafer in-plane uniformity of the doping density, becomes important. In recent years, along with the progress toward a larger-size wafer, the area of a device is also increasing. From such a standpoint, the uniformity of the doping density becomes more important. In the case of an SiC epitaxial film to be formed on the currently mainstream 3- or 4-inch wafer, the in-plane uniformity of the doping density may be from 5 to 10% in terms of standard deviation/average value (σ/mean). However, in the case of the above-mentioned larger size wafer, this value should be reduced to 5% or less.
On the other hand, in the case of the substrate having a size of 3 inch or more, in view of the reduction in the density of defects such as basal plane dislocation, or increase in the yield of a substrate to be produced from an SiC ingot, a substrate having an off-angle of about 4° or less rather than that having a conventional off-angle of 8° has been used. In the case of the epitaxial growth on a substrate having such a small off-angle, the ratio (C/Si ratio) of the number of carbon atoms to the number of silicon atoms in the raw material gas to be flowed during the growth may be generally set to be lower than the conventional ratio. In this connection, as the off-angle becomes smaller, there may be developed a tendency that the number of steps on the surface is decreased and step-flow growth is less liable to occur, so as to cause an increase in the step bunching or epitaxial defects. Accordingly, the above ratio may be set low so as to suppress such a tendency. However, when the C/Si ratio is decreased, so-called site-competition may be noticeable and the introduction of impurity such as nitrogen atoms from the atmosphere may be increased during the epitaxial growth. The thus introduced nitrogen atoms may function as a donor in SiC, and supply electrons, to thereby increase the carrier density. On the other hand, because residual nitrogen is present in the growth atmosphere, the site-competition may occur even in a non-doped layer which has been formed without the addition of an impurity element. Accordingly, the residual carrier density in the case of a non-doped layer which has been grown by decreasing the C/Si ratio may become higher than that in the case of the conventional C/Si ratio. This will be described below by referring to FIG. 1.
In the case of a substrate having a conventional off-angle (8°), the growth thereon may be performed by setting the C/Si ratio to near X, and in this case, the residual carrier density in the non-doped layer is referred to as “NX”. Under this assumption, the residual carrier density in the non-doped layer to be grown at a low C/Si ratio of “Y” (usually, about 1.0), which is required when a film is to be grown on a substrate having an off-angle of about 4° or less, may become “NY” (usually on the order of 0.8 Lo 1×1015 cm3). On the other hand, the carrier level “NC” which is required for the operation of device may be, for example, from 1 to 5×1015 cm−3. This value may be almost equal to the level of NY, and accordingly, when the C/Si ratio is Y, a layer having such a value close to the doping value, which is required for the operation of a device, is already obtained in an undoped state. Accordingly, in a case where nitrogen is intentionally introduced as a doping gas to control the carrier level of the layer so as to provide a value which is required for the operation of a device, the doping amount to be controlled may be smaller. Therefore, it may be difficult to obtain the uniformity of the doping density, as compared with that in the case of a 8°-off substrate. Further, to be exact, the C/Si ratio may not be constant in all of portions on a wafer, and the C/Si ratio may be locally smaller than Y. In this case, as seen from FIG. 1, the residual carrier density may become larger than NC.
FIG. 2a shows a doping density profile when the doping is performed at a C/Si ratio in the portion of Y, and FIG. 2b shows a doping density profile when the doping is performed in the same wafer at a C/Si ratio in the portion of less than Y (approximately from 0.8 to 0.9). If the residual carrier densities at respective portions are referred to as NB1 and NB2, usually, NB1 may be approximately from 0.8 to 1×1015 cm−3, and NB2 may be approximately from 1 to 3×1015 cm−3, and accordingly, a relationship of NB1<NB2=about NC is established. If the doping is performed so that NC can be obtained in the portion of FIG. 2a in the wafer, the doping amount may be NC-NB1 and therefore, the doping amount in the portion of FIG. 2b may necessarily become NC-NB1+NB2. Accordingly, NB2-NB1 may be doping variation in the portions of FIGS. 2a and 2b, and can be a value which is larger than about 10% of NC. Such a phenomenon may be produced because the graph in FIG. 1 shows a large gradient at a C/Si ratio of near Y, which is required for the growth on a substrate having an off-angle of 4° or less. That is, despite a small variation of the C/Si ratio, the value of NB2-NB1 becomes large in the vicinity of Y, to thereby provide a large reduction in the uniformity of the in-plane distribution of the doping density.
Therefore, in an SiC epitaxial growth substrate, the application of which to a device may be expected in the near future, the wafer in-plane uniformity of the doping density may be deteriorated, due to the fact that when the off-angle of the substrate is changed from conventional 8° to about 4° or less, the growth therefor should be performed by reducing the C/Si ratio, and this may be a problem for the application thereof to a device.
In this connection, the present inventors have proposed a method for forming a high-quality epitaxial film on an SiC single crystal substrate having an off-angle of 4° or less, wherein a layer which has been grown by setting the ratio of the numbers of carbon atoms and silicon atoms contained in the material gas for the epitaxial film (C/Si) to be 0.5 or more and less than 1.0 (defect-reduced layer), and a layer which has been grown by setting C/Si to be 1.0 or more and 1.5 or less (active layer) are disposed (see Patent Document 1). However, this method has a purpose of obtaining an epitaxial film which has been reduced in the triangular epitaxial defect or in the surface roughening, and but does not teach the means for directly securing the uniformity of the doping density of the epitaxial film in the wafer plane.